H-Bridge: Black Box or Are Details Important?

Engineers in all disciplines use electronics in their designs for sensing, actuation, and real-time control. Today there are few exceptions. A common component is an operational amplifier (op-amp), which most engineers treat as a black box containing many transistors and resistors. Its performance is primarily determined by the components (e.g., resistors and capacitors) surrounding it as long as the op-amp has negative feedback and its limitations are not exceeded.

Another common electronic component is the H-bridge. Any engineer who has ever controlled a motor has most likely used the H-bridge, but perhaps treated it as a black box with no thought as to how it works and how it affects overall system performance. The H-bridge needs to be thoroughly understood for model-based design and optimum system performance.

The H-bridge, as shown in the diagram, is named because of its configuration. It has four switching elements (transistors, MOSFETs are a good selection, p-channel for high side, and n-channel for low side) with the load (usually brushed DC or step motor) at the center. The diodes are of the Schottky type with short turn-on delay. The four transistors can be turned on and off independently. If transistors 2 and 3 are turned on, the motor turns in one direction. Turn transistors 1 and 4 on and the motor turns in the opposite direction. The transistors are usually controlled in a pulse-width modulated (PWM) fashion. When the transistor is on, it behaves like a small temperature-dependent resistor -- the lower the value the better for heat dissipation.

When the transistor is completely off, it conducts no current. MOSFETs are voltage-driven devices. The gate forms a parasitic capacitor with the source, and this capacitance limits the speed at which the transistor can be turned on and off. In the transitional periods, the power dissipation due to switching is significant, especially when the switching frequency is higher than a few hundred hertz (Hz). The role of the diodes is often overlooked and they are intrinsic in MOSFETs. While the bridge is on, two of the four transistors carry the current and the diodes have no role.

However, once the bridge is turned off, the transistors will not conduct current. When the load is inductive, as with motors, the electromagnetic field associated with it will collapse when the transistor is turned off and the diodes provide a low-resistance path for that current to flow and thus keep the voltage on the motor terminals within a reasonable range. The dissipated heat from the diodes can be of the same order of magnitude as the heat dissipation from the transistor switching.

The load (motor) is modeled as an inductor (Lm), resistance (Rm), and speed-dependent voltage (back emf) in series, the values of which are all dependent on motor rotor position. The motor torque is proportional to the current flowing through this series combination. There are two extremes. When the motor runs with no load, the current is low and the motor terminal voltage is close to the back-emf voltage. When the motor is stalled, the back-emf voltage is zero and the motor acts like an inductor.

The H-bridge can be driven in many different ways. In general, the on-time behavior is rather simple: Turn on one high-side transistor and the opposite low-side transistor to allow current to flow through the motor. It is the off-time drive that makes the difference. Since transistors 1 and 2 (or 3 and 4) should never be turned on at the same time, there are only three different combinations for those two switches: Transistor 1 conducts, or transistor 2 conducts, or neither conducts. There are many different drive modes. Andras Tantos has provided an excellent, detailed explanation, and I highly recommend it.

A black box approach to some commonly used devices is justified, but the H-bridge is not one of them.

Adding regeneration to any motor driving controller leads to another challenge, because aside from the additional controlling requirements there is now a need to put all of that energy someplace, which is no small matter. Most power supplies are single-quadrent devices, and so they can't handle it, a load resistor needs to be able to dissipate all of that power without failing, and a battery power source can only accept charge at some maximum rate. So the addition of regeneration adds a lot to the design effort.

And if you sometimes want to see just how much energy is stored in a rotating motor, just run one up to speed and then quickly disconnect the power source and short circuit the motor leads. But make sure that the motor can't roll off the bench before you do this, because most motors will attempt to roll over. It has always been an interesting demonstration for first year electrical technology students.

tain yes, using graphics for the display of dircuits is certainly a lot more convenient, and able to provide a lot more details as well. The only reason that we used the text method was that then anybody with any operating system, and any file viewing program could read the circuit diagram. This was back in the day when ms was not the dominant OS provider, back before gates killled Dr.Dos and the other OS companies. At one time we did have a choice with computer OSs.

a.saji, in a system that uses bipolar transistors as part of the higher current circuitry the saturation voltage drop often causes a larger amount of power dissipation than can be provided without additional heat sinking. While bipolar transistors can sometimes provide an advantage in these applications, a low effective "on" resistance is not one of those advantages. Thus a heat sink is sometimes required.

Battar, you are certainly correct about switching speeds and generating EMI. That is always the challenge, since faster switching reduces power dissipation during the transition while it increases the high frequency components. The expensive solution is to use filtering and shielding, both of them add to cost without improving performance. So most designs are some form of compromise, which goes along exactly as you stated.

I agree, but IC is not always the solution sometimes for your custom application you require a custom H Bridge, for that its important to know the in depth knowledge of H bridge, what parameters to select in it etc.

For both the H-Bridge devices and op-amps, the intense pain can come from ignoring some details. Little things like maximum slew rate or input current versus any of a number of variables. And on H-Bridges, switching speed and drive requirements versus switching speed are the sort of thing that one might sometimes be lucky and get away with ignoring. But sometimes not. And unfortunately the higher the required performance the more that the details must be dealt with. That is the inconvenient reality. In some low performance applications one can sometimes get away without considering those details.

Industrial workplaces are governed by OSHA rules, but this isn’t to say that rules are always followed. While injuries happen on production floors for a variety of reasons, of the top 10 OSHA rules that are most often ignored in industrial settings, two directly involve machine design: lockout/tagout procedures (LO/TO) and machine guarding.

Load dump occurs when a discharged battery is disconnected while the alternator is generating current and other loads remain on the alternator circuit. If left alone, the electrical spikes and transients will be transmitted along the power line, leading to malfunctions in individual electronics/sensors or permanent damage to the vehicle’s electronic system. Bottom line: An uncontrolled load dump threatens the overall safety and reliability of the vehicle.

While many larger companies are still reluctant to rely on wireless networks to transmit important information in industrial settings, there is an increasing acceptance rate of the newer, more robust wireless options that are now available.

To those who have not stepped into additive manufacturing, get involved as soon as possible. This is for the benefit of your company. When the new innovations come out, you want to be ready to take advantage of them immediately, and that takes knowledge.

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